This invention relates to friction paper feeders and, in particular, friction paper feeders used to supply either originals or copy sheets to an electrostatic copier.
Various arrangements of friction rollers or belts have been used in an attempt to insure the reliable feeding of a sheet from a stack while at the same time preventing the feeding of more than one sheet at a time. One type of paper feeder of the prior art, operating on the differential friction principle, employs a driven feed roller opposing a retarding roller driven in an opposite direction at their point of contact. The feed roller surface has a relatively high coefficient of friction with paper, while the retarding roller surface has a coefficient of friction with paper less than that of the feed roller but greater than that between two successive sheets of paper.
In order for feeders of this type to operate satisfactorily, the coefficient of friction of the feed roller with paper must always exceed that of the retarding roller, which in turn must always exceed the coefficient of friction between two sheets of paper. After some period of use, however, even rollers having a high initial coefficient of friction become coated with fibers from the paper and their coefficient of friction drops down to about unity. Since the coefficient of friction between successive sheets of paper can be as high as 0.7, there is very little latitude for permissible variation in the coefficient of friction of the retarding roller and unreliable operation may result.
In still another type of friction feeder known to the art, rather than having two rollers of different coefficients of friction, opposing rollers both having high coefficients of friction are used. The feed roller is positively driven in a forward direction, but the retarding roller, rather than being driven at a constant reverse velocity, is subjected to a predetermined reverse torque. The retarding roller is free, however, to rotate in a forward direction if the externally applied torque is sufficient to overcome this predetermined torque. The predetermined torque applied to the retarding roller is selected so that it is sufficient to separate two sheets of paper in the nip but is insufficient to overcome the frictional force between either roller and a contacting sheet of paper.
Thus, if only a single sheet of paper is presented to the roller nip, the feed roller not only advances the sheet of paper but also overcomes the predetermined torque and causes the retarding roller to rotate in a forward direction. If, however, two sheets of paper are presented to the roller nip, the feed roller will continue to advance the first sheet, but the reverse torque applied to the retarding roller separates the two sheets of paper and thereafter moves the second sheet rearwardly out of the nip.
Osgood et al. U.S. Pat. No. 2,892,629 shows a feeder of this type using a torsion spring in combination with a friction clutch to bias the retarding roller using energy derived from the feed roller through frictional engagement. Van Dalen et al. U.S. Pat. Nos. 3,272,500, Breuers 3,044,770, and Gibson 4,060,232 show similar arrangements in which the retarding roller is driven from an independent energy source through a friction clutch which decouples at the desired level of torque.
Feeders of this type have the advantage over differential friction feeders that the coefficient of friction of the retarding roller can be as high as practical and need not be less than the friction of the feed roller. The only constraint on the coefficients of friction is that each roller have a coefficient of friction with paper that is greater than the coefficient of friction between two sheets of paper. Because of this relaxed requirement, the reliability of feed is substantially increased.
One drawback shared by feeders of the type disclosed in the Osgood et al. patent, in which a spring cocked by the feed roller supplies the reverse torque to the retarding roller, is the dependence of the reverse torque on the degree to which the spring is wound. Because of this dependence, which is generally linear, some period will elapse following initial actuation of the rollers before the spring is sufficiently tensioned to supply the desired torque. If two or more sheets enter the roller nip before this period has elapsed, unreliable operation may result. Any attempt to shorten the initial period by lowering the spring compliance will effect a corresponding shortening of the "throw" of the retarding roller at the desired reverse torque, also leading to unreliable operation.
Another drawback, shared by all of the feeders disclosed in the above-identified patents, is the dependence of the reverse torque on the frictional characteristics of the friction clutches used. As the working surfaces become worn, their frictional characteristics may change, and operation may become unreliable.